The major histocompatibility antigens (MHC antigens) are molecules involved in self-nonself differentiation in the defense mechanism of the living body against infection. They are classified into Class I molecule composed of .alpha. chain and .beta.2M, and class II molecule composed of .alpha. chain and .beta. chain. A groove for trapping an antigen peptide is present on the .alpha.1 and .alpha.2 domains, and also on the .alpha.1 and .beta.1 domains. They are featured to have the T cell receptor recognize only a fragmented peptide trapped in the groove, thereby achieve cell death (cellular immunity) by CD8+ cells which have recognized the class I antigens, as well as induce mainly antibody production (humoral immunity) by CD4+ cells which have recognized the class II antigens.
The MHC genes constitute a gene group most full of polymorphism, and the locations of pockets, shapes, sizes and properties of the peptide trapping grooves are different among haplotypes. It is considered that association conditions of the trapped fragment peptides may vary depending on these differences, which decide immune response and disease sensitivity of each individual. The correlation between the MHC haplotypes and a resistance to a disease (disease insusceptibility) or a possibility of the onset of a disease (disease susceptibility) has been reported, for example, as to human immune deficiency virus (HIV), human T cell leukemia virus (HTLV) and malaria.
As for the bovine MHC (BoLA) class II genes, existence of DQA, DQB, DRA, DRB, DNA, DOB, DYA, and DYB genes has been estimated. DRB3, inter alia, which is one of the three genes (DRB1 to B3) identified on the DRB genetic locus, has been known to encode a functional protein, and existence of 73 alleles has been revealed so far. However, there is almost no report about correlation between bovine infectious diseases and the bovine MHC (BoLA) haplotypes.
In particular, as to the bovine leukemia virus (BLV), which has the gene PX that regulates virus proliferation in the same manner as the human immunodeficiency virus (HIV) and is a retrovirus most related to HTLV-I, a research group in the United States has reported its relationship with the bovine MHC (BoLA) haplotypes mainly focusing on disease resistance; however, its relationship with possibility of onset of leukemia has not been reported. The ratio of cattle infected by this virus (infection rate in Japan) is 10-20%, and 1-2% of the infected cattle develop extremely malignant endemic bovine leukemia and die after a long latent period of 10-15 years. Therefore, economic loss of stockbreeders caused by the virus is very serious. If a possibility of the onset of leukemia in cattle after BLV infection can be evaluated by the analysis of bovine MHC (BoLA) haplotypes, it becomes possible to preliminarily select disease resistant cattle for breeding, and it is expected that extremely safe cattle breeding can be continued.
Accordingly, an object of the present invention is to elucidate the relationship between the bovine leukemia virus (BLV) and the bovine MHC (BoLA) haplotypes, and to provide a method for convenient judgement of a possibility of the onset of leukemia of a cattle caused by the bovine leukemia virus (BLV) and a resistance to the onset of the leukemia by means of genetic engineering techniques. Another object of the present invention is to provide a primer set useful for the aforementioned method for judgement.